Modelling sediment process behaviour of headwater stream channels using terrain parameters and associated relationships

Abstract

Erosion of soil and inundation by sediment of productive land is a major problem for New Zealand primary industry. Disruption of the soil resource through erosion results in subsequent reduction of production potential of the landscape. Accelerated sediment supply to drainage networks can also reduce channel capacity and increase the likelihood of flooding. The aim of this thesis is to develop a system for classification of terrain types in order to predict active fluvial sediment processes in headwater streams. A four stage empirical methodology is employed to develop the system. The nature of the contemporary sediment process regime operating in a representative catchment is assessed. The catchment is then delineated into zones of like process using the concept of the fluvial network being a continuum. A number of terrain parameters are evaluated for each of these zones which allows verification that the identified zones are different in terms of morphometry and morphology. A simplification of this methodology is developed which relates easily recognisable and characteristic combinations of terrain parameters to the active sediment process regime. The study site used is the Arai Matawai sub-catchment in the southern part of Waipaoa Catchment, Gisborne, New Zealand, which is considered representative of the Te Arai Landsystem. In this catchment seven fluvial sediment process regimes are identified and mapped as Sediment Process Zones. These include: High Magnitude Sediment Supply Zones; Moderate Magnitude Sediment Supply Zones; Sediment Supply and Transport Zones; Sediment transport Zones; Zones of Sediment Transport with some Storage; Periodically Evacuated Sediment Storage Zones; and, Sediment Storage Zones. Each zone is found to be distinguishable in terms of channel bed material, channel and valley magnitude (as described by width and order), position in the catchment, and channel gradient. Channel bed material, channel and valley floor width, and position in the catchment are recognised as being the most easily identifiable of the parameters without the use of specialised technology. Characteristic combinations of these parameters are presented that are indicative of the active process regime. Using this methodology a detailed study of the fluvial process regime need only be undertaken once, in a representative catchment, for each identified landsystem type. From this a simple model is developed that can be used for fast classification of the remainder of the area of concern. This system is developed for use on a time scale concordant with contemporary planning frameworks rather than for assessing process changes on a 'geological' or 'geomorphological' time scale (that is, periods of tens of years rather than hundreds or thousands of years). If a time period is to be placed on the applicability of this model it is related to the occurrence of high magnitude formative erosion events that have major effects on the sediment process regime of fluvial networks.